Planetary gear trains are complex mechanisms for reducing, or occasionally increasing the rotational speed between two rotating shafts or rotors. The compactness of planetary gear trains makes them appealing for use in aircraft engines where space is at a premium.
The forces and torque transferred through a planetary gear train place tremendous stresses on the gear train components, making them susceptible to breakage and wear, even under ideal conditions. In practice, conditions are often less than ideal and place additional stresses on the gear components. For example the longitudinal axes of a planetary gear train's sun gear, planet carrier, and ring gear are ideally coaxial with the longitudinal axis of an external shaft that rotates the sun gear. Such perfect coaxial alignment, however, is rare due to numerous factors including imbalances in rotating hardware, manufacturing imperfections, and transient flexure of shafts and support frames due to aircraft maneuvers. The resulting parallel and angular misalignments impose moments and forces on the gear teeth, the beatings which support the planet gears in their carrier, and the carrier itself. These imposed forces and moments accelerate gear component wear and increase the likelihood that a component will break in service. Component breakage is obviously undesirable in any application, but particularly so in an aircraft engine. Moreover, accelerated component wear necessitates frequent inspections and part replacements which can render the engine and aircraft uneconomical to operate.
The risk of component breakage can be reduced by making the gear train components larger and therefore stronger. Increased size may also reduce wear by distributing the transmitted forces over correspondingly larger surfaces. However increased size offsets the compactness that makes planetary gear trains appealing for use in aircraft engines, and the corresponding weight increase is similarly undesirable. The use of high strength materials and wear resistant coatings can also be beneficial, but escalates the cost of the gear train and therefore does not diminish the desire to reduce wear.
Stresses due to misalignments can also be reduced by the use of flexible couplings to connect the gear train to external devices such as rotating shafts or nonrotating supports. For example, a flexible coupling connecting a sun gear to a drive shaft flexes so that the sun gear remains near its ideal orientation with respect to the mating planet gears even though the axis of the shaft is oblique or displaced with respect to a perfectly aligned orientation. Many prior art couplings, however, contain multiple parts which require lubrication and are themselves susceptible to wear. Prior art couplings may also lack adequate rigidity and strength, with respect to torsion about a longitudinal axis, to be useful in high torque applications. Misalignment can also be accommodated by a splined connection. However the motion that occurs between the contacting spline teeth in a splined connection creates friction which is highly variable and causes the flexibility of such a connection to also be variable.
In view of these shortcomings a simple, reliable, unlubricated coupling system for connecting components of a planetary gear train to external devices while accommodating misalignment therebetween is sought.